FLUVIAL PALEOENVIRONMENTS IN THE INTRACRATONIC AMAZONAS BASIN (EARLY MIOCENE - EARLY MIDDLE MIOCENE, COLOMBIA)

Based on a detailed study of Early Miocene to early Middle Miocene sediments in the Caqueta River area (Amazonas Basin, Colombia), a reconstruction of the palaeoenvironmental and palaeogeographical history is made. Sediment composition, sedimentary sequences and transport directions indicate that the succession was deposited by a low sinuosity fluvial system with an anastomosing character that originated from the Guyana Shield. Basin subsidence, Precambrian and Palaeozoic palaeorelief, and a changing base level controlled deposition in this fluvial system. Sedimentary environments such as channels, abandoned channels, crevasse splays, backswamps and palaeosoils characterized this system where fluvial flooding was common. The Early Miocene to early Middle Miocene sequences have been truncated by a more recent fluvial system, probably of Pliocene to Pleistocene age, representing the precursor of the present Caqueta River. Palynological analysis suggest that the sediments are part of the Retitricolporites and the Psiladiporites-Crototricolpites pollen zones. Fifteen new species are described which belong to the genera: Psilatricolpites, Retitricolpites, Retitricolporites, Bombacacidites, Syncolporites and Psilastephanocolporites. The palynological assemblages indicate that the area was characterized by palm swamps, riparian vegetation and a rather diverse tropical forest. Intervals rich in marine palynomorphs accompanied by high concentrations of mangrove pollen grains (Zonocostites) suggest periods of marine influences and mangrove development. It is thought that the marine intervals are related to the Late Burdigalian transgression.